CO2 Batteries That Store Grid Energy Take Off Globally

_{This giant bubble on} the island of Sardinia holds 2,000 tonnes of carbon dioxide. But the gas was not captured in factory emissions, nor extracted from the air. It comes from a gas supplier and lives permanently inside the dome system for an environmentally friendly purpose: storing large amounts of excess renewable energy until it is needed.

Developed by Milan-based Energy Dome, the bubble and its surrounding machinery feature a one-of-a-kind “CO2 battery,” as the company calls it. The installation compresses and expands the CO₂ daily in its closed system, turning a turbine that generates 200 megawatt hours of electricity, or 20 MW over 10 hours. And in 2026, replicas of this plant will start appearing all over the world.

We mean that literally. It only takes half a day to inflate the bubble. Construction of the rest of the facility takes less than two years and can be done just about anywhere there is 5 acres of flat land.

The first to build one outside Sardinia will be one of India’s largest electricity companies, NTPC Limited. The company plans to complete its CO2 battery by 2026 at the Kudgi Power Plant in Karnataka, India. In Wisconsin, the utility Alliant Energy received approval from authorities to build one in 2026 to power 18,000 homes.

And Google likes the concept so much that it plans to quickly roll out the facilities to all of its key data centers in Europe, the United States and the Asia-Pacific region. The idea is to provide energy-hungry data centers with clean energy 24 hours a day, even when the sun isn’t shining or the wind isn’t blowing. The partnership with Energy Dome, announced in July, marked Google’s first investment in long-duration energy storage.

“We traveled the world looking for different solutions,” says Ainhoa ​​Anda, Google’s senior manager of energy strategy, in Paris. The challenge the tech giant has faced is not only finding a long-lasting storage option, but also one that is compatible with each region’s unique specifications. “So standardization is really important, and that’s one of the things we really value” at Energy Dome, she says. “They can really plug and play this.”

Google will prioritize placing Energy Dome installations where they will have the most impact on decarbonization and grid reliability, and where there is plenty of renewable energy to store, Anda says. Facilities can be placed next to Google’s data centers or elsewhere in the same grid. The companies did not disclose terms of the deal.

Anda says Google hopes to help the technology “reach a massive commercial stage.”

Getting creative with long-lasting energy storage

All of this excitement is based on the full-scale, grid-connected Energy Dome power plant in Ottana, Sardinia, which was completed in July. It was built to help solve one of the biggest challenges of the energy transition: the need for grid-scale storage capable of providing electricity for more than 8 hours at a time. Called long-duration energy storage, or LDES in industry parlance, this concept is the key to maximizing the value of renewable energy.

When sun and wind are abundant, solar and wind farms tend to produce more electricity than a grid needs. So it makes sense to store the excess for use when these resources are scarce. LDES also makes the grid more reliable by providing backup and supplemental power.

The problem is that even the best new grid-scale storage systems on the market (primarily lithium-ion batteries) only offer about 4-8 hours of storage. That’s not long enough to operate for an entire night, or several cloudy, windless days, or during the hottest week of the year when energy demand peaks.

Once the CO2 leaves the dome, it is compressed, cooled, reduced to a liquid and stored in pressure vessels. To release the energy, the process is reversed: the liquid is evaporated, heated, expanded, then introduced into a turbine which generates electricity. Luigi Avantaggiato

The size of lithium-ion battery systems could be increased to store more and last longer, but systems of this size are generally not economically viable. Other grid-scale battery chemistries and approaches are under development, such as sodium, iron-air, and vanadium-based redox flow batteries. But energy density, costs, degradation and financing complications have challenged developers of these alternatives.

Researchers have also experimented with energy storage by compressing air, heating blocks or sand, using hydrogen or methanol, pressurizing water at depth, and even dangling heavy objects in the air and dropping them. (The creativity devoted to LDES is impressive.) But geological constraints, economic viability, efficiency, and scalability have hampered the commercialization of these strategies.

The proven grid-scale storage option – pumped hydropower, in which water is pumped between reservoirs at different altitudes – lasts for decades and can store thousands of megawatts for days. But these systems require specific topography, a lot of land and can take up to a decade to build.

CO2 batteries tick a lot of boxes that other approaches don’t. They do not require special topography like pumped hydroelectric reservoirs do. They do not need critical minerals like electrochemical and other batteries do. They use components for which supply chains already exist. Their expected lifespan is almost three times that of lithium-ion batteries. And adding size and storage capacity to them significantly reduces the cost per kilowatt hour. Energy Dome expects its LDES solution to be 30% cheaper than lithium-ion.

China took note. China Huadian Corp. and Dongfang Electric Corp. would be building a CO plant₂energy storage facility based in the Xinjiang region of northwest China. Media reports show renderings of domes but give widely varying storage capacities, including 100 MW and 1,000 MW. Chinese companies did not respond IEEE Spectrumrequests for information from .

“What I can say is that they are developing something very, very similar [to Energy Dome’s CO2 Battery] but on a fairly large scale,” says Claudio Spadacini, founder and CEO of Energy Dome. Chinese companies “are good, they’re super fast and they have a lot of money,” he says.

Why is Google investing in CO2 batteries?

When I visited the Energy Dome facilities in Sardinia in October, the CO₂ had just been pumped out of the dome, so I was able to take a look inside. It was huge, monochromatic and almost empty. The internal membrane, which retained the uncompressed CO₂had collapsed all over the floor. A few pockets of gas remained, causing the off-white sheet to swell in places.

Meanwhile, the translucent exterior dome let in some daylight, creating a creamy glow that enveloped the vast space. With no structural framework, the only thing keeping the dome upright was the small pressure difference between the inside and outside air.

“It’s amazing,” I told my guide, Mario Torchio, Energy Dome’s global director of marketing and communications.

“It’s true. But it’s physics,” he said.

Outside the dome, a series of machines connected by undulating pipes moves the CO₂ out of the dome for compression and condensation. First, a compressor pressurizes the gas from 1 bar (100,000 pascals) to approximately 55 bars (5,500,000 pa). Then a thermal energy storage system cools the CO₂ at room temperature. Then a condenser reduces it to a liquid that is stored in a few dozen pressure vessels, each the size of a school bus. The whole process takes around 10 hours, and at the end the battery is considered charged.

To discharge the battery, the process is reversed. Liquid CO₂ is evaporated and heated. It then enters a gas expansion turbine, which is similar to a medium pressure steam turbine. This drives a synchronous generator which converts mechanical energy into electrical energy for the network. After that, the gas is vented at ambient pressure to the dome, filling it while waiting for the next charging phase.

Energy Dome engineers inspect the drying system, which maintains the CO₂ gas in the dome at optimal dryness levels at all times.Luigi Avantaggiato

It’s not rocket science. Still, someone had to be the first to put it together and figure out how to do it profitably, which Spadacini says his company achieved and patented. “The way we seal turbomachines, the way we store heat in thermal energy storage, the way we store heat after condensation… can really reduce costs and increase efficiency,” he says.

The company uses pure, specially designed CO₂ instead of getting it from emissions or air, as these sources contain impurities and moisture that degrade the steel in machinery.

What happens if the dome is breached?

In contrast, the Energy Dome installation takes up about twice as much land as a lithium-ion battery of comparable capacity. And the domes themselves, which are about the height of a sports stadium at their tops or more, could stand out in a landscape and provoke some cringe.

What if a tornado came? Spadacini says the dome can withstand winds of up to 160 kilometers per hour. If Energy Dome can have a half-day warning of extreme weather conditions, the company can simply compress and store the CO.₂ in the tanks, then deflate the outer dome, he said.

If the worst happens and the dome is breached, 2,000 tonnes of CO₂ will enter the atmosphere. This is equivalent to the emissions of around fifteen round-trip flights between New York and London on a Boeing 777. “It’s negligible compared to the emissions from a coal-fired power plant,” explains Spadacini. People will also need to stay 70 meters or more away until the air clears, he said.

Is the risk worth it? The companies lining up to build these systems seem to think so.